Tool to measure the latency of touchscreen devices

ABSTRACT

In an embodiment, a latency measuring head is provided for use in measuring touch-to-response latency in a test device, the test device including a capacitive user interface that responds to touch input. The latency measuring head includes a conductive element adapted to be positioned in static proximity with and/or in contact with the capacitive user interface. An electron sink is operatively connected to the conductive element via a normally open switch having an open and a closed position. The electron sink has capacity to hold or dissipate a sufficient charge to trigger a touch event on the test device when the switch is closed. A photosensitive element is positioned in static proximity with and/or in contact with the capacitive user interface such that the photosensitive element can output a signal in response to a change in an optical property of at least a portion of the capacitive user interface. Software for performing analysis of latency measurements from a latency measuring device and generating statistics that summarize latency performance of a test device is further disclosed.

This application is a non-provisional of, and claims priority to, U.S.Provisional Patent Application No. 62/233,064 filed Sep. 25, 2015, theentire disclosure of which is incorporated herein by reference.

This application includes material which is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent disclosure, as it appears in thePatent and Trademark Office files or records, but otherwise reserves allcopyright rights whatsoever.

FIELD

The present invention relates in general to the field of touchscreendevices, and in particular to latency measuring tools for measuringlatency in capacitive touchscreens.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of the invention will be apparent fromthe following more particular description of preferred embodiments asillustrated in the accompanying drawings, in which reference charactersrefer to the same parts throughout the various views. The drawings arenot necessarily to scale, emphasis instead being placed uponillustrating principles of the invention. Although example embodimentsand associated data are disclosed for the purpose of illustrating theinvention, other embodiments and associated data will be apparent to aperson of skill in the art, in view of this disclosure, withoutdeparting from the scope and spirit of the disclosure herein.

FIG. 1 is a perspective view (left) and a bottom view (right)illustrating an embodiment of a disclosed latency measuring device.

FIG. 2 is a block diagram illustrating an embodiment of a disclosedlatency measuring device.

FIG. 3 shows a chart illustrating raw and clustered latency data.

FIG. 4 shows a table illustrating software delay validation results.

FIG. 5 shows a chart illustrating raw results of 600 latencymeasurements on a touchscreen smart phone.

FIG. 6 shows a table illustrating latency measurements for existingdevices based on 500 measurements.

DETAILED DESCRIPTION

In an embodiment, a latency measuring head is provided for use inmeasuring touch-to-response latency in a test device, the test deviceincluding a capacitive user interface that responds to touch input. Inan embodiment, the latency measuring head includes a conductive elementadapted to be positioned in static (i.e., at a fixed distance) proximitywith and/or in contact with the capacitive user interface. In anembodiment, an electron sink is operatively connected to the conductiveelement via a normally open switch having an open and a closed position.In an embodiment, the electron sink has capacity to hold or is adaptedto dissipate a sufficient charge to trigger a touch event on the testdevice when the switch is closed. In an embodiment, a photosensitiveelement is positioned in stationary (i.e., at a fixed distance)proximity with and/or in contact with the capacitive user interface suchthat the photosensitive element can detect, and output a signal inresponse to a change in an optical property of at least a portion of thecapacitive user interface.

In an embodiment, a latency measuring device as disclosed hereinprovides a cost-effective yet high-accuracy and high-precision automatedtool that measures the interface latency of touchscreen devices. Thedevice can directly measure latency by triggering a capacitive touchevent on a test device using an electrically actuated touch simulatoralong with a photosensitive element to monitor the screen for a visualresponse. This allows measurement of full end-to-end latency of atouchscreen system exactly as it would be experienced by a user. In anembodiment, the latency measuring device can be configured so that itdoes not require human interaction to perform a measurement, thus,enabling the acquisition of large datasets. Also disclosed are a seriesof tools and equipment to assess and validate the performance of thelatency measuring device, and demonstrate that it can provide reliablelatency measurements.

With reference to FIG. 1, in an embodiment, the disclosed latencymeasuring device comprises a microcontroller 1 and a measurement head 2that rests on a device under test 3. The measurement head 2 includes aphotosensitive element 4 to measure display an optical property and aconductive element 5 to trigger touches on capacitive screen. Thephotosensitive element 4 may be, for example, a photodiode, a camera, aphotoresistor, a phototransistor, a CCD, or a CMOS. As would beunderstood by one of ordinary skill in the art in view of thisdisclosure, the aforementioned list of potential photosensitive elementsis not exhaustive and is not limited solely to the items listed in theaforementioned example. In an embodiment, an optical property can beused as one basis of measurement. The optical property may bebrightness, contrast, color, luminance, or other optical property. In anembodiment, the conductive element 5 is a brass contact. In anembodiment, the conductive element comprises any conductive material,including but not limited to brass, aluminum, bronze, iron, steel,nickel, tin, silver, gold, platinum, carbon, conductive plastics, indiumtin oxide, or any material with a high dielectric. As would beunderstood by one of ordinary skill in the art in view of thisdisclosure, the aforementioned list of conductive materials for theconductive element is not exhaustive and is not limited to only thosematerials listed in the aforementioned example. In an embodiment, a goalof the latency measuring device is to provide a reliable apparatus toaccurately measure end-to-end interface latency on touchscreen devices.This means estimating the time from touch activation to the display ofthe corresponding visual response. This interval is caused by bothsoftware and hardware components. Such software and hardware componentsinclude components along the following flow of events: the capacitivetouch sensor of the test device detects the touch; the touch informationis captured by the hardware and processed by the operating system; amessage is passed to the active application; the application updates itsinternal state and draws new information to a display buffer; thedisplay buffer is passed to the operating system; the operating systemasks the display to render the display buffer; the display updates itspixels, and is able to finally show the user the feedback for the touchactivation. Within this flow, each step contributes to the finalend-to-end latency. (In an embodiment, as discussed in more detailbelow, a goal of the latency measuring device is to provide a reliableapparatus to accurately measure component parts of the end-to-endinterface latency on touchscreen devices.)

Latency is not constant and may depend on numerous factors, thus, somevariability is to be expected. While the input sensor and the displayare scanned and updated cyclically (typically between 60 and 120 Hz,with constant data transfer costs), the touch sensing is not alwayssynchronized with the finger landing on the display surface. Inaddition, system load is not always consistent, and the display is notalways ready to render when a buffer swap is requested. Moreover,latency is strongly connected to the display refresh rate. On a 60 Hzdisplay, new frames may be displayed every 16.7 ms. Minor improvementsin latency may be absorbed by the refresh rate and large increases inlatency may suddenly occur if a small additional delay forces thegraphical update to wait for the next display frame.

Although latency measurements can be based on timestamp informationinternal to a device, both software and hardware factors affectingperformance suggest that the most accurate measurement of latency isusually taken by 1) optically observing the screen's response to aphysical input, and 2) running a series of trials to cope withmeasurement variations, as described above. The present disclosurefocuses on a device that measures end-to-end latency—from activation toobservable visual response—and that may automate multiple measurementcycles, thereby reducing human intervention (e.g., only required forinitial setup for the measurement session).

An example of use and operation of the presently disclosed latencymeasuring device is as follows. In an embodiment, the latency measuringdevice is placed on top of a touchscreen device running an applicationthat responds to a touch event by causing a significant change in anoptical property, such as brightness. In an embodiment, prior to eachmeasurement session, the latency measuring device measures the lightlevels associated with the bright “on” and dark “off” states of thescreen and calibrates brightness thresholds for each state. A hostcomputer controls the latency measuring device, and an operator caninitiate multiple runs of unattended measurements. The latencymeasurements are automatically clustered and analyzed; the results areprovided to the operator in both graphical and textual formats.

In an embodiment, when a measurement cycle is initiated, the latencymeasuring device records a timestamp and uses a touch simulator totrigger the capacitive screen and generate a touch event. This may beentirely automated and does not require human interaction, except e.g.,to initiate the sequence. In an embodiment, from the touchscreen's pointof view, the simulated touch is perceived as a human touch on thescreen. In an embodiment, the simulated touch is indistinguishable froma human touch. After initiating the touch, the latency measuring deviceuses a photosensitive element such as a photo sensor or photodiode towatch for the change in an optical property (e.g., screen brightness,contrast, color, or luminance) caused by the response to the touch. Oncea change in optical property is observed, the latency measuring devicerecords a second timestamp. The elapsed time between the timestampsconstitutes a single latency measurement. The automated nature of thelatency measuring device means that hundreds of measurements can easilybe collected when evaluating a device. In an embodiment, the value ofthe first timestamp is associated with the time of triggering, and thesecond timestamp is associated with the time the measurable change inoptical properties occurs. In an embodiment, the first and/or secondtimestamp may be adjusted for latency or lag between the recording of atimestamp and the events with which a timestamp is associated.

In an embodiment, a custom application is used in order to ensuremaximum, predetermined or otherwise quantifiable changes in opticalproperty (e.g., brightness, contrast, color, or luminance). In anembodiment, the custom application may, for example, draw a black screenand then draw a white region in response to a touch event. However, inan embodiment, it is possible to use any change in interface brightness(e.g., a button activation highlight) at the cost of some sensitivity.This allows measurement of the latency of commercial applications thatdo not permit third-party code and/or on devices that do not permitthird-party code. It also allows measurement of the latency ofcommercial applications and/or devices without the use of third-partycode. In an embodiment, an application running on the test device can beconfigured to output other types of responses from the device inaddition to screen response. Such other types of response include, e.g.,electrical, acoustic, and tactile responses.

In an embodiment, the latency measuring device has no moving parts inthe touch simulator. Design challenges associated with a latencymeasuring device having no moving parts are detailed below. In anembodiment, solving the design challenges allows reproducibility and theability to precisely trigger touch events without human intervention. Inan embodiment, removing the human element from the measurement processenabled automation, which is a significant contribution to the latencymeasuring device.

FIG. 2 shows a block diagram illustrating main components of the latencymeasuring device in accordance with an embodiment. The main boardtriggers a touch by closing the switch 1 to generate a capacitivedisturbance on the screen. The visual response is monitored by thephotosensitive element 2 connected to the microcontroller. In anembodiment, the computed latency is then output to the host computer. Inan embodiment, a time associated with the switch closing and a timeassociated with a responsive change are then output to a host computer.In an embodiment, the measurement head contains the touch actuator(using a conductive element such as a brass contact approximately thesize of a human finger tip) and photosensitive element. When positionedon top of a touchscreen, the conductive element is designed so that itwill not trigger a touch by itself. However, in an embodiment, theconductive element is electrically connected to an electron sink (e.g.,an electron sink that provides additional mass and/or surface area), andthus triggers a touch event. In an embodiment, an electronicallyactuated normally open (NO) switch controls this connection. When theswitch is closed, the conductive element is connected to the electronsink, disturbing the capacitive levels on the touchscreen and triggeringa touch event.

Implementing the latency measuring device may involve providingsolutions for one or more of the steps defined in the previous section:generating a touch event, capturing feedback from the display, andaccurately calculating the time between the two events. Moreover, thereare some practical considerations (e.g., holding the latency measuringdevice to the device under test) that influence the design.

In the following examples of the implementation of an embodiment of alatency measuring device, we begin with a discussion of the measurementhead, which, in this embodiment, is responsible for generating touchesand sensing the screen's response, and then proceed with a discussion ofthe main board, which houses the microcontroller. As would be understoodby one of ordinary skill in the art in view of this disclosure, the mainboard, microprocessor, and other components described herein may beseparate from or integral with the measurement head depending upon theparticular application without departing from the spirit and scope ofthe invention.

Measurement Head

In an embodiment, the measurement head is a roughly cylindricalenclosure that includes the circuitry to generate the simulated touchand deliver it to the screen through a conductive element (e.g., a brasscontact) as well as a photosensitive element to sense feedback (See FIG.1, right). Such enclosure may be custom 3D printed or mass manufactured.In an illustrative embodiment, the conductive element and photosensitiveelement (e.g., photodiode, camera, photoresistor, phototransistor, CCD,or CMOS) protrude through a hole in the bottom of the head, and rest onthe screen during testing. In an illustrative embodiment, the remainderof the bottom surface is covered in a layer of EPDM rubber (or a similarsubstance) to allow the head to grip the screen. In an illustrativeembodiment, a small circuit board is mounted inside the head andcontains the switch (and supporting circuitry) that generates the touch;this switch is discussed in detail in the following section. In anillustrative embodiment, the top of the head is covered in Velcrohook-and-loop fastener, which enables repositionable weights that aidthe positioning of the measurement head such that it remains flat on thescreen. As described below, two main functions of the measurement headare generating touch and sensing feedback.

Generating Touch

One goal of a latency measuring device is to reduce the introduction ofhuman-induced errors during measurement. To effectively remove humaninteraction from the measurement loop (e.g., to reduce the introductionof human-induced errors), the latency measuring device can create asimulated touch that is indistinguishable from an actual human touch. Inorder to do this, the latency measuring device generates a change in thescreen capacitance that the sensor identifies as a touch, and, in anembodiment, it is able to programmatically trigger and reverse thischange.

In an embodiment, to generate changes in capacitance, the conductiveelement is connected to an electron sink. Brass may be selected as thematerial for the conductive element because it is highly conductive,corrosion resistant, and relatively soft (as compared to other metals)reducing the likelihood of screen damage due to scratches; it is alsorelatively lightweight, which reduces the likelihood of screen damagedue to impacts. In an illustrative embodiment, the conductive element is12 mm in diameter (similar to a fingertip) and 2 mm tall. Designparameters for the conductive element are quite flexible but, in anembodiment, care must be taken to design the element does not trigger atouch event on its own. In an embodiment, the conductive element has asmall enough surface area so that the element does not trigger a touchevent on its own. In an illustrative embodiment, the inside of theconductive element (i.e., the side that is facing into the measurementhead enclosure) was coated in liquid rubber insulation.

In an embodiment, the conductive element is connected to an electronsink in order to dissipate additional charge, nonetheless, unintentionaldissipation is avoided. In an illustrative embodiment, connecting theconductive element to one end of a 30 cm 22 AWG wire with alligatorclips (leaving the other end of the wire disconnected) was sufficient toinduce a touch event. In an embodiment a small bare aluminum heat sink(Ohmite R2V-CT6-38E) is employed. As will be apparent to a person ofskill in the art in view of this disclosure, the characteristics of thealuminum heat sink (conductive material arranged to maximize its surfacearea) are consistent with the design requirements of an electron sink asrequired herein. In an embodiment, connecting the contact to theelectron sink permitted touch to be reliably triggered. In anembodiment, as discussed in more detail below, the touch triggering isprogrammatically enabled and disabled by the connection anddisconnection of the electron sink and the conductive element.

Triggering touch programmatically: A very small change in capacitance(on the order of 1 pF) is needed to register a touch event on acapacitance sensor. Accordingly, in an embodiment, a switch with highlyinsulated mechanical relay is used. In an embodiment, although it iselectrically actuated, the switch selected must have very small leakagebecause, as will be apparent to a person of skill in the art in view ofthis disclosure, a very small change in capacitance can trigger thetouch response. The leakage in many switches result in a touch event assoon as the latency measuring device comes in contact with the screen,regardless of the switch position. During the development processnumerous alternatives were examined and tested, including solid-stateanalog switches, tri-state devices, transistors, solid-state relays, andmechanical relays. In an embodiment, acceptable performance was observedusing an Omron G6J-2P-Y, a highly insulated mechanical relay. In anembodiment, a low leakage switch, along with such careful hardwaredesign as will be apparent to a person of skill in the art in view ofthis disclosure, can mitigate and/or eliminate erroneous touch events.In an embodiment, a transistor, diode, an analog switch, a MEMS switch,or any low-capacitance switch may be used to electrically connect anddisconnect the electron sink from the conductive element.

As will be apparent to a person of skill in the art, a reliance on amechanical switch may introduce problems. For example, switching amechanical relay involves energizing a coil to magnetically move a metalcontact, and such a movement (e.g., in the macroscopic world) takes farlonger—on the order of several milliseconds—than the sub-atomicequivalent in a solid-state switch. While one could potentially ignore afew nanoseconds of solid-state switching time, millisecond-scaleswitching times cannot be ignored when taking millisecond-scale latencymeasurements. As a result, in an embodiment, a double pole relay withtwo switches controlled by a single coil was used: one pole was used toswitch the touch simulator, and the other pole was fed into an input pinon the microcontroller. In such configuration, a change on that inputpin indicated that the relay had closed, allowing the system to accountfor the relay's switching time (e.g., removing or subtracting it fromthe latency measurement). Depending on a selected switch, the bounceassociated with the closure of a mechanical switch could be of concernbecause physical contacts can vibrate and cause a short period of rapidoscillations when a switch closes. In an illustrative embodiment, bouncewas measured with an oscilloscope over a dataset of 50 relay closings,and an average bounce time of 0.118 ms (SD=0.003) was determined. In anembodiment, where bounce time is low, it can be ignored. In anembodiment, bounce time must be accounted for in the system. As would beunderstood by one of ordinary skill in the art in view of thisdisclosure, any type of switch that functions with the circuitrydisclosed herein may be implemented, without departing from the scopeand spirit of the disclosure herein.

As will be apparent to one of skill in the art in view of thisdisclosure, when designing the circuitry to trigger touch, attention ispaid to minimize current leakage and stray capacitance, since thesecould otherwise trigger unintended touches. In an embodiment, a relay isacting as a switch between the conductive element (which is alwaysresting on or in close proximity to the screen) and the electron sink,one half of the switch will always be “connected” to the conductiveelement, regardless of the switch position. In an embodiment, the lengthof the connection between the switch and the conductive element isminimized. In an embodiment, the length of the connection between theswitch and the conductive element is selected to avoid accidental orunintended touch. In an embodiment, a relay is placed in close proximityto the conductive element in the measurement head. In an embodiment, arelay is placed as close as possible to the conductive element in themeasurement head. In an embodiment, a small circuit board is used insidethe head to allow the relay to be placed in close proximity to theconductive element.

Sensing Feedback

In an embodiment, to compute a test device's latency, the latencymeasuring device identifies feedback generated by the simulated touch.Screen response can be identified as a change in optical property. In anembodiment, screen response to touch is a change from a black screen toa white one. In an embodiment, a Vishay BPW46 PIN photodiode can be usedto identify changes in brightness. The Vishay BPW46 PIN photodiode has afast and consistent response time, a flat-sided package that can sitflush against the screen, and good visible light sensitivity.

In an embodiment, the photosensitive element can be mounted in themeasurement head adjacent to the conductive element, which allows it toobserve a response in close proximity to the touch event and isconnected to an ADC (analog-to-digital converter) in the microcontrollervia a shielded cable to the main board.

In an embodiment, a raw analog signal is transmitted over the cable. Araw analog signal may be sensitive to noise. In an embodiment, no activecircuitry is located adjacent to the photosensitive element in themeasurement head. In an embodiment, a high-speed ADC is located in themeasurement head to allows transmission of a digital signal from themeasurement head to the main board, making the signal more resilient tonoise. In an embodiment, noise may be mitigated by shielding. In anembodiment, the effect of noise may be mitigated in software (e.g.,detecting and rejecting anomalous readings or measurements from thephotosensitive device (e.g., the photodiode) or anomalous measurementsbased on such anomalous readings or measurements.

Main Board

With the conductive element and photosensitive element in place, themain board is next described. In an embodiment, the main board containsthe microcontroller and supporting circuitry; it is responsible fortriggering the switch to generate a touch, and then sensing thephotosensitive element for changes in the optical property being sensed.In an embodiment, when a change in optical property is detected, thelatency measuring device can calculate the time difference and output alatency measurement. In an embodiment, the latency measuring device canoutput time measurements of starting and ending events instead of, or inaddition to the time difference or latency measurement. As noted above,the main board, microprocessor, and other components described hereinmay be separate from or integral with the measurement head dependingupon the particular application.

In an embodiment, the logic and computing functions of the latencymeasuring device are provided by a microcontroller board. Examples ofsuitable microcontroller boards include, e.g., the Arduino Duemicrocontroller board, which is based on an Atmel SAM3X8E ARM Cortex-M3CPU running at 84 MHz. In addition to the digital I/O pins, the latencymeasuring device can make use of one of the built-in 12-bit ADCs to readthe photodiode light levels. The customized supporting circuitry for thelatency measuring device is built on a protoshield, which provides 30cm² of perfboard in an Arduino-compatible form factor. In an embodiment,the measurement head is connected to the main board via a pair ofshielded Category 6 STP cables (i.e., Ethernet cable and connectors).The cables do not carry an Ethernet signal; rather, they are simplyreadily available shielded multi-conductor cables with built-inconnectors. In an embodiment, the latency measuring device may sendmeasurements or components of the measurement (e.g., timestamps) to thehost computer, and these measurements or components of the measurementmay be logged/aggregated by the host. In an embodiment, the latencymeasuring device itself may aggregate a plurality of measurements orcomponents of the measurement (e.g., timestamps). Examples of softwarefor a microcontroller that may be used in connection with theabove-defined measurement apparatus, and a host computer that may beused in connection with the above-defined measurement apparatus, as wellas device-specific helper applications are described in the nextsection.

Latency Measuring Device Software

In an embodiment, several software components are required to perform alatency measurement. The main responsibility of the software is tocoordinate the events defined in the previous sections and to store thatinformation in the host computer for later analysis. For convenienceherein, software is divided in two main components: firmware and hostcomputer software, although it will be apparent to one of skill in theart in view of this disclosure that the activities of either may besupplanted, or supplemented, by the other, or by software running onadditional or different hardware. In an embodiment, software that runson the device under test is provided. The practitioner can provide“white touch” applications for several platforms that draw a blackscreen and then respond to a touch event by drawing a white region, butany application which causes a change in optical property, such as abrightness change at (or in the vicinity of) the point of a touch (or atanother location), will suffice.

In an embodiment, the latency measuring device may be configured tomeasure end-to-middle, middle-to-middle and middle-to-end latency inaddition to, or in lieu of end-to-end latency. In accordance with suchembodiment, the latency measuring device breaks down individual parts ofthe test device's performance. For example, the test device can beconfigured with software to output an electrical signal when it hasidentified a touch but before it has fully processed it. In anembodiment, in response to that signal, and the latency measuring devicecan be configured to record three timestamps: the onset of touch (asbefore), the time it received a signal indicating touch capture (new),and the display response (as before). In an embodiment, many suchsignals are output, and allowing the latency measuring device to producea finer grained picture (e.g., one or more middle-to-middle latencymeasurements) of where the device latency originates from sinceintermediate timestamps can be assigned to individual components of thedevice's response.

In another embodiment, the latency measuring device and the device undertest synchronize a clock. Using their common clocks, the two devicesrecord timestamps associated with a variety of events. Thus, instead ofoutputting a signal during the test, the device under test may reportrecorded information that occurred during the test. The recorded clockinformation from one or both devices is then used to determinepoint-to-point latency between any two events for which clockinformation has been recorded.

Firmware

In an embodiment, firmware is responsible for the low-level actionsinvolved in running a latency measurement and for transmitting theresults back to the host computer. It can also run a series of preciselyspaced measurements, which is used by the host software to createextended latency datasets; the data flow is described in the followingsection. Finally, the firmware may also be responsible for maintaining acalibration of brightness levels (or other optical property levels) ofthe “on” and “off” touch states. In an illustrative embodiment, firmwareis written in an Arduino variant of C, and interacts with the hostcomputer via a USB-based serial console. In an embodiment, the protocolis human-readable to facilitate easy debugging operations.

Host Computer Software

In an embodiment, host software is the front end of the latencymeasuring device. In an illustrative embodiment, it is an interactivecommand line application written in Python that sends commands to thefirmware, receives and analyses the results, and outputs data files andcharts to the user.

Dataflow

A sample dataflow of an illustrative system for latency measurement isas follows: The user places the latency measuring device on the screenof the device under test, and indicates the desired number of series torun, each of which consists of 10 measurements. Extended testing coulduse 100 series (for a total of 1000 measurements), but a small number ofseries (e.g., 5) may yield a good overview of the device's latency. Thehost software instructs the firmware to calibrate the “on” and “off”light levels and then begins running the first series of tests. Betweeneach series, a randomized delay between 0-16.7 ms is communicated to thelatency measuring device in order to distribute the measurements acrossthe screen's refresh cycle. When all series are complete, the user isalerted, and results are shown on-screen. During the capture of latencymeasurements, user interaction is only required if the latency measuringdevice has detected a malfunction (e.g., a human is required to push apower button to wake the device). Once all of the data is collected, thehost software runs a k-means clustering algorithm across all of themeasurements from all of the series. This is done because groupingsbased on the display refresh cycle are expected. The output datatypically cluster around multiples of display refresh times. The raw andclustered data is then logged, and a visual representation of theobserved latencies is displayed. Raw data is also made available to theuser for logging or further analysis.

A sample chart illustrating output from the above sample dataflow isshown in FIG. 3. In particular, FIG. 3 shows a graphical summarygenerated by an embodiment of the disclosed latency measuring toolshowing 100 latency measurements of a Google Nexus 9 tablet. The datasetcontains 10 series, each of which contains 10 measurements. Each rowcorresponds to one series. Measurements are clustered into two groups,with means spaced one display frame (16.8 ms) apart, illustrating thevariability of the device's latency.

Validation

The development of any type of test equipment requires a careful andthorough validation process to ensure that the equipment is, in fact,measuring reality. In many cases it is possible to verify new testequipment against an existing gold standard or test regime, providing achain of metrological traceability. However, there is no standard orregime for latency measurement devices. Disclosed herein is a series oftests and test equipment to provide confidence that the latencymeasuring device is correctly measuring latency.

The first test measures relative latency by intentionally adding latencyto a device and checking that the latency measuring device is able todetect it. The second test measures the overhead involved in a latencymeasuring device measurement by creating and measuring a zero-latencydevice. The third test seeks to validate the absolute accuracy andrepeatability by creating a simple oscillator-based latency generatorcircuit and simultaneously measuring it with the latency measuringdevice and with an oscilloscope. The fourth and final testcross-validates the latency measuring device by building a camera-basedapparatus and simultaneously measuring a real device with both thecamera and the latency measuring device.

Software Delay Test

Two variants of the above-referenced “white touch” applications werecreated (running on iOS and Android, respectively) that added anarbitrary delay prior to drawing the white region in response to atouch. Each application added this delay with a different programmaticmechanism. As a first measurement of the device's accuracy, the addedlatency is measured. This allows verification of the relativemeasurement of performance of the latency measuring device (e.g.,configurations A and B are 50 ms apart) independent of the latencymeasuring device's absolute measurement performance (e.g., configurationA has 100 ms of latency).

The first version introduces milliseconds of latency (usingPOSIX_(usleep)). Several series of latency measurements were run usingdifferent added latencies using an iPad mini running iOS 7.1.2. Thesecond application delays the response by a certain number of displayframes, rather than by a certain number of ms. By doing so it waspossible to take into account the device's limitation in rendering tothe display and measure latency in display frames. To add additionalrobustness to the verification, Android was used for this test, andAndroid's Choreographer was used to schedule the screen update in thefuture. Tests were performed on a Nexus 9 running Android 5.1.1. FIG. 4shows software delay validation results for both iPad and Nexus 9 tests.Each row shows one test condition containing 200 measurements. The tableshows data for the two largest clusters (Main and Secondary) whichconstitute almost all measurements. The right column shows thedifference between the baseline and the Main cluster, which matches theexpected value. Secondary clusters are one 60 Hz display frame slower.

Overhead Test

In an embodiment, the latency measuring device measures absolute levelsof latency. An issue in calibrating these measurements is to understandthe overhead of the measurement process. Several steps in themeasurement process have small but finite durations (e.g., the timerequired for the ADC to read the photosensitive element's signal). In anembodiment, care is taken to ensure that these steps do not introduce anuncorrected delay that would skew the latency measurements. To measurethis overhead, a device configuration was created that mimicked azero-latency device by simply displaying a white screen, thus, thelatency measuring device would detect a visual touch response as soon asit is able to check for one, and any reported non-zero latency wouldtherefore entirely consist of measurement overhead.

In connection with an illustrative embodiment, twenty samplemeasurements revealed a consistent overhead of 0.006 ms (SD=0 ms), whichis several orders of magnitude smaller than the quantities that arebeing measured. In an embodiment, measurement overhead that is severalorders of magnitude smaller than the quantities that are being measuredmay be ignored. In an embodiment, measurement overhead that is severalorders of magnitude smaller than the quantities that are being measuringmay be accounted for in the latency measurements.

Measuring Latency in the Real-World

The same device can routinely exhibit a wide range of latencies,sometimes varying by as many as 3 frames (50 ms) between sequentialsamples, as exemplified in FIG. 5. FIG. 5 shows raw results of 600latency measurements on a touchscreen smart phone, a Google Nexus 5running Android 5.1.1. The device exhibits a latency profile thatchanges over time.

This phenomenon may not occur on specialized devices such as thedemonstrator systems described previously in the literature, but it isevident and widespread on commercially-available devices.

Some of the observed factors impacting performance are:

Heat. On many systems, as their internal temperature increases, the OSmay choose to disable or throttle one or more CPU cores to reduce thethermal load. This effect is more pronounced on small devices (e.g.,phones or tablets), since they often lack fans or other active thermalmanagement technologies and thus have fewer options to cool themselves.The effect of thermal throttling during longer measurement runs on somedevices were observed. For example, on the dataset from a Nexus 5 phoneshown in FIG. 5, the initial sets of measurements were split betweenclusters at 69.1 and 85.2 ms. However, after approximately 300measurements, an additional frame of latency would suddenly appear, andlatency clusters at 100.6 and 117.4 ms were recorded. Allowing the phoneto cool off (or briefly placing it in a freezer) would restore thefaster performance.System Load. As one would expect, a busy OS handling a large number ofprocesses will perform worse than a system that is idle. Reducing thenumber of background tasks (e.g., quitting other apps, disabling radiosvia Airplane mode, etc.) would often improve device latency.Battery State. Similar to heat, throttling may occur if the device'sbattery level is low, and the OS switches off or throttles CPU cores inan attempt to save power.Vertical Screen Refresh. As discussed above in the section on the directoptical latency measuring apparatus, most screens redraw from top tobottom. This redraw time is small, but can be on the order of ms. Forexample, it was observed that a Nexus 9 could take upwards of 10 ms toredraw its screen. This means that the location of the measurement headcan have an impact, since the same redraw operation will appear to takeless time if the screen response is measured at the top rather than thebottom of the screen. It may therefore be necessary to standardize alocation (e.g., the center of the screen) for the measurement head. Themeasurements discussed herein were obtained from the center of thescreen, a position we assume is centered in the redraw step.

Latency in general can be thought of as a range of possible values,rather than a single number. However, all of the above variables can bequantified, and can often be obtained programmatically by themeasurement applications (e.g., querying the OS to determine the currentpower saving mode), thus adding context to the obtained measurements.Furthermore, the automation facilitated by the latency measuring devicedescribed herein allows consideration of latency within the context ofthe complexity of real-world devices in real-world conditions.

Various embodiments may be designed to meet various goals in accordancewith the particular application. The following discussion presentsexamples of goals which may be useful for a particular embodiment, but,as would be understood by one of skill in the art in light of thisdisclosure, are not exhaustive or required to practice the techniquesdisclosed herein. These goals relate to precision and accuracy, range,flexibility, invasiveness, and automatic operation.

In an embodiment, with respect to precision and accuracy, given thelower bounds of latency perception, a measuring tool should be accurateand repeatable to within 1 ms. In an embodiment, with respect to range,some commercially available systems have latencies well over 100 ms,while some research-based systems have latencies of less than 1 ms. Inan embodiment, a measuring tool should be able to report a wide range oflatencies. In an embodiment, with respect to cost, the requiredequipment and infrastructure should be inexpensive, costing, forexample, less than $100 USD. With respect to flexibility, in anembodiment, the tool should be able to measure any capacitivetouchscreen device, regardless of size or platform. In an embodiment,with respect to invasiveness, the tool should be able to measure deviceswhether or not the tester has the ability to change or instrument theoperating system, or even the ability to install application software onthe device. In an embodiment, with respect to automatic operation, thetool should be capable of taking measurements without any humaninvolvement after the initial setup and should be able to take a largenumber of unattended measurements within a short timespan in order togenerate statistically significant results.

FIG. 6 shows a table illustrating latency measurements for existingdevices based on 500 measurements. This table shows examples of latencymeasurements for commercial devices. The latency is reported for the twolargest clusters characterized by their size. As in the table of FIG. 4,the two largest clusters of measurements are presented. Five hundredmeasurements were taken for each device. As discussed above, a number offactors can impact the latency of a device. While we have attempted tocontrol for many of these (e.g., low temperature, low load, fullbattery), others, such as differences in software, are beyond ourcontrol. As such, it is important to understand that the data presentedin FIG. 6, while accurate and representative, do not lend well tocross-device comparisons.

Throughout this disclosure, the terms “touch”, “touches,” “contact,”“contacts”, or other descriptors may be used to describe events orperiods of time in which a user's finger, a stylus, an object or a bodypart is detected by the sensor. In some embodiments, these detectionsoccur only when the user is in physical contact with a sensor, or adevice in which it is embodied. In other embodiments, the sensor may betuned to allow the detection of “touches” that are hovering a distanceabove the touch surface or otherwise separated from the touch sensitivedevice. The use of language within this description that impliesreliance upon sensed physical contact should not be taken to mean thatthe techniques described apply only to those embodiments; indeed,generally, what is described herein would apply equally to “contact” and“hover” sensors, each of which being a “touch” as that term is usedherein. More generally, as used herein, the term “touch” refers to anact that can be detected by the types of touchscreens disclosed herein.

The present systems and methods are described above with reference tocomputing devices and microcontrollers devices. It is understood thatsuch computing devices and microcontrollers may be implemented by meansof analog or digital hardware and computer program instructions. In thisrespect, computer program instructions may be provided to a processor ofa general purpose computer, special purpose computer, FPGA, ASIC, SoC,or other programmable data processing apparatus, such that theinstructions, which execute via a processor of a computer or otherprogrammable data processing apparatus. Except as expressly limited bythe discussion above, in some alternate implementations, thefunctions/acts noted in the blocks may occur out of the order noted inthe operational illustrations.

At least some aspects disclosed can be embodied, at least in part, insoftware. That is, the techniques may be carried out in a specialpurpose or general purpose computer system or other data processingsystem in response to its processor, such as a microprocessor, executingsequences of instructions contained in a memory, such as ROM, volatileRAM, non-volatile memory, cache or a remote storage device. Functionsexpressed in the claims may be performed by a processor in combinationwith memory storing code and should not be interpreted asmeans-plus-function limitations.

Routines executed to implement the embodiments above may be implementedas part of an operating system, firmware, ROM, middleware, servicedelivery platform, SDK (Software Development Kit) component, webservices, or other specific application, component, program, object,module or sequence of instructions referred to as “computer programs.”Invocation interfaces to these routines can be exposed to a softwaredevelopment community as an API (Application Programming Interface).Such computer programs typically comprise one or more instructions setat various times in various memory and storage devices in a computer,and that, when read and executed by one or more processors in acomputer, cause the computer to perform operations necessary to executeelements involving the various aspects.

A machine-readable medium can be used to store software and data whichwhen executed by a data processing system causes the system to performvarious methods. The executable software and data may be stored invarious places including but not limited to ROM, volatile RAM,non-volatile memory and/or cache. Portions of this software and/or datamay be stored in any one of these storage devices. As would beunderstood by one of ordinary skill in the art in view of thisdisclosure, the aforementioned list of potential storage places for theexecutable software or data, or portions thereof, is not exhaustive andis not limited to the locations listed in the aforementioned example.Further, the data and instructions can be obtained from centralizedservers or peer-to-peer networks. Different portions of the data andinstructions can be obtained from different centralized servers and/orpeer-to-peer networks at different times and in different communicationsessions or in a same communication session. The data and instructionscan be obtained in entirety prior to the execution of the applications.Alternatively, portions of the data and instructions can be obtaineddynamically, just in time, when needed for execution. Thus, it is notrequired that the data and instructions be on a machine-readable mediumin entirety at a particular instance of time.

Examples of computer-readable media include but are not limited torecordable and non-recordable type media such as volatile andnon-volatile memory devices, read only memory (ROM), random accessmemory (RAM), flash memory devices, floppy and other removable disks,magnetic disk storage media, optical storage media (e.g., Compact DiskRead-Only Memory (CD ROMS), Digital Versatile Disks (DVDs), etc.), amongothers.

In general, a machine readable medium includes any mechanism thatprovides (e.g., stores) information in a form accessible by a machine(e.g., a computer, network device, personal digital assistant,manufacturing tool, any device with a set of one or more processors,etc.)

In various embodiments, hardwired circuitry may be used in combinationwith software instructions to implement the techniques. Thus, thetechniques are neither limited to any specific combination of hardwarecircuitry and software nor to any particular source for the instructionsexecuted by the data processing system.

As used herein, and especially within the claims, ordinal terms such asfirst and second are not intended, in and of themselves, to implysequence, time or uniqueness, but rather are used to distinguish oneclaimed construct from another. In some uses where the context dictates,these terms may imply that the first and second are unique. For example,where an event occurs at a first time, and another event occurs at asecond time, there is no intended implication that the first time occursbefore the second time. However, where the further limitation that thesecond time is after the first time is presented in the claim, thecontext would require reading the first time and the second time to beunique times. Similarly, where the context so dictates or permits,ordinal terms are intended to be broadly construed so that the twoidentified claim constructs can be of the same characteristic or ofdifferent characteristic.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A latency measuring head for use in measuringtouch-to-response latency in a test device, the test device including acapacitive user interface that responds to touch input, the latencymeasuring head comprising: a conductive element adapted to be positionedin a first fixed proximity to the capacitive user interface, which thefirst fixed proximity may include being in contact with the capacitiveuser interface; an electron sink operatively connected to the conductiveelement via a normally open switch having an open and a closed position,the electron sink being adapted to cause dissipation of sufficientcharge to trigger a touch event on the test device in response to theswitch being closed; and a photosensitive element adapted to bepositioned in a second fixed proximity to the capacitive user interface,which the second fixed proximity may include being in contact with thecapacitive user interface, the photosensitive element being furtheradapted to output signals corresponding to at least one optical propertyof at least a portion of the capacitive user interface.
 2. The latencymeasuring head according to claim 1, further comprising amicrocontroller adapted to close the normally open switch.
 3. Thelatency measuring head according to claim 2, wherein the microcontrolleris further adapted to measure a time period from a time at which thenormally open switch is closed to a time when there is a change in anoptical property of at least a portion of the capacitive user interface.4. The latency measuring head according to claim 2, wherein themicrocontroller is further adapted to: a. identify a first time, thefirst time corresponding to a time at which the normally open switch isclosed; b. receive the signals corresponding to at least one opticalproperty; c. identify a second time, the second time corresponding to atime at which there is a predetermined change in the received signals;and, d. use the first time and the second time to compute end-to-endlatency of the test device.
 5. The latency measuring head according toclaim 2, wherein the microcontroller is under control of a hostcomputer.
 6. The latency measuring head according to claim 1, whereinthe latency measuring head is under control of a host computer.
 7. Thelatency measuring head according to claim 6, wherein the latencymeasuring head is further adapted to communicate with the host computerso as to enable the host computer to: a. identify a first time, thefirst time corresponding to a time at which the normally open switch isclosed; b. receive the signals corresponding to at least one opticalproperty; c. identify a second time, the second time corresponding to atime at which there is a predetermined change in the received signals;and, d. use the first time and the second time to compute end-to-endlatency of the test device.
 8. The latency measuring head according toclaim 1, wherein the conductive element comprises a conductive material.9. The latency measuring head according to claim 1, wherein theconductive element comprises at least one material selected from thegroup: brass, aluminum, bronze, iron, steel, nickel, tin, silver, gold,platinum, carbon, conductive plastics, indium tin oxide, or a materialwith a high dielectric.
 10. The latency measuring head according toclaim 1, wherein the photosensitive element is adapted to output signalscorresponding to at least one optical property on only a portion of thecapacitive user interface.
 11. The latency measuring head according toclaim 1, wherein the photosensitive element is adapted to output signalscorresponding to at least one optical property at any location on thecapacitive user interface.
 12. The latency measuring head according toclaim 4, wherein the photosensitive element comprises a photodiode andwherein the predetermined change in the received signals is apredetermined change in brightness.
 13. The latency measuring headaccording to claim 1, wherein the photosensitive element comprises atleast one device selected from the group: photodiode, camera,photoresistor, phototransistor, charge-coupled device (CCD), orcomplementary metal-oxide-semiconductor (CMOS).
 14. The latencymeasuring head according to claim 1, wherein the conductive element andthe photosensitive element are in a single housing.
 15. The latencymeasuring head according to claim 1, wherein the conductive element andthe photosensitive element are each in separate housings.
 16. Thelatency measuring head according to claim 2, wherein the microcontrolleris adapted to perform unattended batch collection of latencymeasurements of the test device.
 17. The latency measuring headaccording to claim 16, wherein the microcontroller is adapted to performunattended successive determinations of latency of the test device. 18.The latency measuring head according to claim 2, wherein themicrocontroller is adapted to estimate a time from touch activation todisplay of a corresponding visual response.
 19. The latency measuringhead according to claim 18, wherein the estimation is based upon atleast one timestamp generated external to the test device.
 20. Thelatency measuring head according to claim 1, further comprising afixture adapted to bias a surface of the measurement head towardscontact with a screen of the test device.
 21. The latency measuring headaccording to claim 20, wherein the fixture comprises at least oneselected from the group: repositionable weight, repositionable clamp,strap, magnet, suction cup, vacuum, or adhesive.
 22. The latencymeasuring head according to claim 1, adapted to operate with a customapplication running on the test device.
 23. The latency measuring headaccording to claim 1, adapted to operate without a custom applicationrunning on the test device.
 24. The latency measuring head according toclaim 22, wherein the custom application is configured to display on thetest device a first screen with a region having an optical property at afirst level in the absence of a touch event and to display on the testdevice a second screen wherein the region displays the optical propertyat a second level in response to a touch event.
 25. The latencymeasuring head according to claim 1, wherein the normally open switchcomprises an electrically actuated switch, and the latency measuringhead is adapted to initiate latency measurement without human contactwith any portion of the latency measuring head.
 26. The latencymeasuring head according to claim 1, wherein the switch comprises anelectrically actuated switch and the latency measuring head is adaptedto initiate latency measurement without contact or interaction with ahost computer.
 27. The latency measuring head according to claim 2,wherein the normally open switch is a switch selected from the group:highly insulated mechanical relay, transistor, diode, analog switch,MEMS switch, or low-capacitance switch.
 28. The latency measuring headaccording to claim 26, wherein the normally open switch is a switchselected from the group: highly insulated mechanical relay, transistor,diode, analog switch, MEMS switch, or low-capacitance switch.
 29. Thelatency measuring head according to claim 1, wherein the opticalproperty is at least one optical property selected from the group:brightness, contrast, color, or luminance.
 30. The latency measuringhead according to claim 1, further adapted to measure a time period froma time at which the normally open switch is closed to a time when thereis a change in an optical property of at least a portion of thecapacitive user interface.
 31. The latency measuring head according toclaim 1, further adapted to perform unattended batch collection oflatency measurements of the test device.
 32. The latency measuring headaccording to claim 30, further adapted to perform a plurality ofunattended batch collections of latency measurements.
 33. The latencymeasuring head according to claim 1, further adapted to comprise a fullor partial user interface.
 34. The latency measuring head according toclaim 1, further adapted to estimate a time from touch activation todisplay of a corresponding visual response.
 35. The latency measuringhead according to claim 33, wherein the estimation is based upon atleast one timestamp generated external to the test device.
 36. Thelatency measuring head according to claim 1, wherein the first fixedproximity and the second fixed proximity are different.
 37. The latencymeasuring head according to claim 1, wherein the first fixed proximityand the second fixed proximity are the same.